高压磨料水射流加工中材料去除机理研究

本文根据脆、塑性断裂理论,通过高压纯水射流和磨料水射流对石材冲蚀试验,对冲蚀凹坑成型机理进行了研究。结果表明,高压纯水射流只有不发散的中心射流对材料具有去除作用。高压磨料水射流对材料表面冲击时形成"倒钟"型孔,材料去除机理是在成穴力、剪切应力和水楔的共同作用下,以脆性和塑性断裂方式实现去除。磨料水射流对材料的冲蚀区分为中心射流区、成穴区和喷砂区,材料的主要体积去除量处在成穴区。磨料水射流加工孔的直径和深度随靶距的增加而增加。磨料水射流从射流中心沿径向至射流边缘流速逐渐降低,磨料浓度逐渐增大,但射流能量最高的部位既不在射流中心也不在磨粒集中的射流边缘,而是在成穴区。     

基于LS-DYNA仿真的射流加工参数分析

目的通过LS-DYNA对磨料射流冲蚀切削进行仿真,研究相关工艺参数对切削参数的影响。方法采用磨料水射流对Al_2O_3陶瓷进行了单点冲蚀仿真和切削仿真研究,其中水和磨料粒子采用SPH方法建模,氧化铝陶瓷工件采用FEM方法建模,并通过SPH-FEM耦合算法,实现射流冲蚀切削过程的仿真。结果分析射流冲蚀过程仿真和切削过程仿真可知,射流加工前期,由于射流中磨粒碰撞与反弹,使壁面成不规则"V"型。初始阶段,切深随计算时间呈线性增加,同时壁面对磨粒产生制约作用,从而使加工处的孔深基本不再增加。由于磨粒在冲蚀处壁面底部的冲蚀作用,使凹坑底部宽度增加并迅速趋于稳定。同时切削仿真与冲蚀仿真也存在一定区别,主要由于切削过程设定了移动速度。结论将仿真结果与实验结果进行比较可知,切削深度随着泵压的增大而成线性增大,切深随磨料流量的增大而增大,随靶距和横移速度的增大而减小。其中切深与磨料流量、靶距、横移速度均为非线性关系,工件最大切深与计算时间不呈线性关系增长。     

基于磨料水射流的C/SiC复合材料切槽加工实验研究

实验采用磨料水射流对C/SiC复合材料进行切槽加工实验研究,探究了加工参数对切割深度的影响规律;分析了切槽形貌、材料损伤形式和切割机理。结果表明,在一定参数范围内,切割深度随射流压力和靶距的增大而增大,随进给速度的增大而减小;造成切槽深度、切缝宽度不均匀的主要原因是磨料水射流的雾化作用和二次冲蚀作用;切割面损伤形式主要为基体微裂纹、纤维断裂、界面层脱粘;材料去除形式为碳纤维和碳化硅基体材料脆性断裂去除;碳纤维对基体裂纹扩展具有一定的阻碍作用。     

Ni60/WC涂层表面圆凹坑皮秒激光加工关键参数研究

目的 优化激光路径填充方式以减少皮秒加工圆凹坑底部的堆积现象,并探究基于该激光路径填充方式的皮秒激光关键参数对Ni60/WC涂层表面圆形凹坑形貌参数的影响规律。方法 采用搭建的紫外皮秒激光微加工平台在Ni60/WC涂层表面加工预先规划的直径为230μm的圆凹坑,通过白光干涉仪测试加工所得圆凹坑的整体三维形貌对圆凹坑底部形貌进行表征。采用同心圆网格复合激光路径填充方式对圆凹坑底部堆积现象进行优化,并通过单因素法分析该路径下皮秒激光关键参数,即加工功率、扫描次数、扫描速度对圆形凹坑深度、直径和圆度系数的影响规律。结果 通过优化的同心圆网格复合激光路径填充方式加工所得圆凹坑材料去除体积为7.59×10⁶μm³,轮廓算术平均高度为21.37μm,对比原始的网格激光路径填充方式,加工的圆凹坑底部无明显堆积;基于此激光路径填充方式,在测试工艺参数范围内,圆凹坑深度、直径和圆度系数随激光功率的增大呈二次函数增大;随着扫描速度的增大,圆凹坑深度、直径呈线性减小,圆度系数呈线性增大;圆凹坑深度、直径和圆度系数随扫描次数的增加均呈线性增大。结论 同心圆网...     

超声振动辅助软性磨料流喷孔光整加工研究

针对软性磨料流加工喷油嘴喷孔过程中磨粒容易团聚堵塞流道的问题,提出利用超声振动产生的空化效应,使团聚的磨粒破碎分离,实现液态载体与磨粒的均匀混合。通过研究超声辅助条件下磨粒的受力、运动以及空化效应对磨粒团的作用,结合试验对比研究在超声振动条件下软性磨料流加工喷孔的入口直径与锥度的变化规律。结果表明:空化效应产生的高能量冲击波可提高加工过程中的磨料压力,将磨粒团分散为单颗磨粒群,提高加工质量与加工效率;随着磨料质量分数与磨粒粒径的增大,喷孔入口直径与锥度增大,磨料对喷孔壁面切削作用加强,材料去除量增加。     

弹性基体软固结磨料磨具的材料去除机理

基于弹性磨具的磨抛工艺为硬脆材料超精密加工效率与加工质量的兼顾平衡提供了新的解决思路,但其磨抛过程的材料去除机理尚未明确。为研究弹性磨抛过程中的材料去除行为,以硅橡胶作为磨具基体材料,混合微米级金刚石磨料制备弹性基体软固结磨料磨具,利用有限元仿真分析方法研究弹性基体软固结磨粒的受力状态,结合接触力学与运动学分析建立考虑单颗磨粒磨损行为与有效磨粒数量的材料去除模型,通过石英玻璃试件的弹性磨抛加工试验验证预测模型的准确性。结果表明：石英玻璃试件的材料去除率随着磨抛压力、主轴转速、磨具偏角的增大而显著增加,而磨料粒径对其影响程度较小;当工艺参数组合为磨料粒径100μm、磨抛压力7 N、主轴转速1 500 r/min、磨具偏角20°时,经60 min磨抛后,工件已加工表面粗糙度由1.069μm降至0.089μm,材料去除率为8.893×10⁸μm³/min;该试验条件下,建立的材料去除模型预测准确度相比Preston经典模型提高36.7%。研究成果可为实现硬脆材料的确定性材料去除提供技术支持和理论依据。     

磨料种类和粒径对钢板表面的形貌影响

本文通过实验分析了磨料水射流加工中磨料种类和粒径对热轧Q235钢板表面形貌的影响。采用高压数控水切割设备进行表面除鳞,实验中采用石榴石和棕刚玉两种磨料,并对不同的磨料粒径60目,80目,120目进行了对比分析,射流压力76 MPa、喷射距离50 mm、磨料体积浓度20%、喷嘴尺寸1 mm保持不变。结果表明,石榴石比棕刚玉的材料去除能力更强,且钢板的表面划痕宽度和长度随着磨料粒径的减小而减小,加工范围随着磨料粒径的减小而增大。     

磨料水射流冲蚀热障涂层实验研究

通过开展磨料水射流冲蚀去除热障涂层的实验研究可知,以低于4 MPa压力的射流垂直冲蚀热障涂层时,其冲蚀孔型为"W"型。单因素实验结果表明:冲蚀孔径及孔深随着加工时间、磨粒浓度的增加而增大,当靶距为15 mm时冲蚀深度最大;热障涂层的材料去除速率比镍基高温合金基体低;冲蚀加工表面质量好,热障涂层与金属基体的界面结合处未发现分层和裂纹现象。     

微磨料水射流加工技术的发展

微磨料水射流加工技术的射流直径在10 μm～100 μm之间,较常规磨料水射流直径(500 μm～1200 μm)小一个数量级,在微加工领域仍保持常规磨料水射流的许多的性能,尤其适宜对硬脆材料、复合材料等难加工材料进行微加工.目前其孔加工精度已达到相当于激光微加工技术的水平.为加深对该新技术的最新发展的理解,本文介绍了微磨料水射流加工技术射流生成方式、装置设计的关键技术、主要参数对加工性能的影响及部分应用示例.最后提出了微磨料水射流加工技术中有待深入研究的工作.     

在役油罐内壁除锈喷嘴外流场数值模拟

针对在役单层油罐改造成双层罐的除锈问题,选用磨料水射流喷嘴为研究对象,使用计算流体力学方法,进行了混合磨料水射流对油罐内壁除锈效果的数值模拟。以射流打击靶面的最大剪切力和冲蚀磨损率作为评价指标,研究结果表明:前混合磨料水射流达到相同的除锈效果相较于后混合磨料水射流所需的射流压力更低,射流入射压力为20 MPa时,除锈效果较理想且具有较好的经济性;剪切力在靶面上存在一个核心作用区,最大剪切力位置不在靶面中心处;冲蚀磨损率随磨料的体积浓度、磨料粒径、磨料密度的增大而增大,而最大壁面剪切力不随其增大而变化。     

微粒子喷丸对靶材残余应力场的影响

为了研究微粒子喷丸多颗粒冲击靶材时的残余应力场,建立了1个中心粒子周边均布6个粒子的7粒子冲击模型。应用有限元软件,对微粒子搭接率、冲击速度、冲击角度、直径等喷丸工艺参数对中心粒子冲击坑中心点处残余应力场的影响进行了仿真研究。结果表明:当搭接率ζ≤0.5时,ζ的变化对残余压应力场深度没有影响,而ζ=0.75是表面残余压应力S_S变化的分界线,只有在ζ＞0.75时,残余压应力场的4个特征值才均随着ζ的增大而增大;增大微粒子直径、减小冲击角度,有利于增大残余压应力场的深度,但同时也会减小残余压应力场的大小。增大微粒子的冲击速度,残余压应力场的深度增大,而残余压应力场的大小呈现一定的波动性,但是两者均在冲击速度为200 m/s时取得最大。     

磨料射流铣削工艺参数优化

目的对表面粗糙度和材料去除率作为输出参数的磨料水射流铣削45#钢过程进行研究,旨在寻找最优加工参数。方法对射流去除材料机理进行了分析,设计并进行了以磨料粒度、射流压力、横向进给距离、靶距为加工工艺参数的田氏正交实验。采用Minitab对不同实验参数组合下磨料水射流加工45#钢的表面粗糙度、材料去除效率进行了数据分析,并从材料去除机理方面,对4种加工工艺参数对于铣削表面质量和材料去除效率的影响程度和影响趋势,以及各因素之间的交互作用进行了分析。结果对射流铣削面表面粗糙度影响较显著的因素是横向进给距离,射流压力次之;对于材料去除效率,磨料粒径的影响最显著,横向进给距离次之。结论综合材料去除效率和表面粗糙度值,选出最优加工参数:磨料粒径2000目,射流压力120~160 MPa,喷嘴横移距离1.0~1.5 mm,靶距约30 mm。     

基于SPH的超音速火焰喷涂WC-12Co粒子速度对其沉积行为的影响

目的 以超音速火焰喷涂过程为基础,探究粒子撞击速度对粒子在基体上沉积行为的影响。方法 应用SPH方法,模拟分析WC-12Co粒子速度在400～800 m/s内,单个粒子在相同基体上的沉积行为。结果 粒子撞击速度与粒子扁平率、粒子基体结合面积、结合方式等有密切关系。随着粒子撞击速度的增加,基坑深度持续增大至最小深度的4.6倍,金属射流对提高粒子扁平化程度及粒子与基体的有效结合面积起到促进作用,总接触面积最大可达到原有效接触面积的2.7倍。撞击速度的提升使得有效塑性应变及应变区域增加,形变区域增大。同时,结合面温升总体增加,增强了粒子与基体的结合条件。沉积过程存在能量耗散,初始能量的提高有利于粒子与基体总能量的增加,强化了压实效应,进一步促进粒子与基体的结合。结论 在数值模拟选取的范围内,超音速火焰喷涂WC-12Co粒子的撞击速度越高,粒子与基体的结合状态越好。     

Influence of kinematic operating parameters on kerf geometry in abrasive waterjet machining of silicon carbide ceramics

Silicon carbide (SiC) is extensively used for manufacturing of highly engineered parts due to its high hardness, low coefficient of friction, wear resistance and high decomposition temperature. However, generating 3D surfaces (e.g. pockets) in such structural ceramics by conventional machining is a difficult task. In this context, abrasive waterjet (AWJ) machining, with its capability to cut any material with low specific cutting forces, seems to be the “ideal” processing technique for such materials; nevertheless machining 3D shapes by AWJ milling is still in its infancy. 3D shapes can be generated by “enveloping” them with successions of jet footprints (kerf geometries) generated by varying the process operating parameters. To enable this, the present work investigates the influence of key kinematic operating parameters (i.e. α-jet impingement angle and v-jet feed rate) on the kerf geometry and its dimensional characteristics. Furthermore, the kerf generation mechanism under multi-pass jet erosion was analysed to get control over erosion depth in multi-pass machining. It was found that by varying α (90°–40°), the symmetric/asymmetric kerf geometry is intimately dependent on the variation of standoff distance (SOD), abrasive particle velocity distributions and their local impact angles accounted across the jet footprint. Variation in v influences the exposure time of material to jet and enhances the erosion capability of abrasives impacting at shallow angle that results in different erosion rates along the kerf profile; this combined effect leads to departure of kerf geometry from simple cosine profile approximation to more elliptical type with the decrease of feed speed. Further, at lower jet feed rates, the depth of erosion increased and the low energy abrasive particles along trailing edge of jet plume get enough time to erode the material that results in variation of slope of kerf walls and hence, overall geometry. Based on these observations, the multi-pass trials showed that the successive passes have to account for both the local impact angles of abrasive particles as well as the actual SOD (SOD+initial kerf depth). In this way, by understanding the influence of key kinematic operating parameters (α and v) on the kerf geometry and its dimensional characteristics, the paper establishes a good basis for developing strategies for controlled 3D AWJ machining of complex shapes.     

Particle velocity models for ultra-high pressure abrasive waterjets

Particle velocity and trajectory in an ultra-high pressure abrasive waterjet (AWJ) are important jet characteristic information required for understanding and modelling the particle erosion process involved in AWJ machining. In this paper, mathematical models for the particle velocity variations across and along an AWJ are developed based on an in-depth understanding of the jet dynamic characteristics from a computational fluid dynamics (CFD) simulation study. The models are then assessed qualitatively and quantitatively. It is found that the model predictions are reasonable and consistent with the data from a verified CFD model with an average percentage error of less than 1%.     

提升磨料水射流抛光去除效率的参数研究

具有独特优势的磨粒水射流抛光技术,由于去除效率低,在加工超精密光学元件中的应用受到了一定的限制。本研究通过实验和仿真,分析了喷嘴直径和射流压力对去除函数、去除效率和确定性修形加工的影响。实验结果表明:喷嘴直径在一定尺寸范围内增加,可以有效地提升去除效率,超过一定尺寸后,去除效率增加减慢,去除函数的宽度变化要远大于喷嘴直径的变化;随射流压力增大,去除函数深度呈指数增加,去除函数轮廓由W形转变为双W形。通过建立回转函数来模拟偏心回转射流的方法,验证了变形后的去除函数在确定性抛光加工中具有很好的适用性,并且该方法可以用在采用垂直射流直接进行超精密表面的确定性抛光加工上。      

Abrasive waterjet cutting of polycrystalline diamond: A preliminary investigation

Although abrasive waterjet (AWJ) machining has been employed in different setups (e.g. through cutting, milling, turning, cleaning) to generate surfaces in various workpiece materials (e.g. metallic alloys, ceramics, composites), up to now there is scarce information on the use of this technology in cutting super-hard materials such as diamond-based materials. The paper reports for the first time on a preliminary study of the capability of AWJ cutting of polycrystalline diamond (PCD) using abrasive media with different hardness, i.e. aluminium oxide (Al₂O₃), silicon carbide (SiC) and diamond. While keeping some operating parameters constant (pump pressure, stand-off distance and size of abrasives) the feed speed has been adjusted to enable full jet penetration for each type of abrasives. It was found that not only the material removal rates vary significantly with the employment of different types of abrasives but also the nozzle wear ratios, with further implications on the kerf quality (width, taper angle) of diamond cut surfaces. Furthermore in-depth studies of the cut surfaces helped to reveal the material removal mechanism when different types of abrasives are employed: Al₂O₃—low intensity erosion; SiC—medium erosion with undesired cracking; diamond—high intensity erosion. The experimental results showed that while Al₂O₃ and SiC abrasive media yield modest material removal rates (comparable with those obtained by electro discharge machining-EDM) the use of diamond abrasives can greatly increase (>200 times) the productivity of AWJ through cutting of PCD test pieces at acceptable roughness (Ra<1.6 μm) and integrity (i.e. crack-free) of the cut surfaces. Despite some limitations (e.g. cost of diamond abrasives, extensive nozzle wear rates) that can be overcome through further developments, it is believed that this preliminary research gives an indication of the capability of the AWJ to profile diamond-based structures for high-value engineering applications where conventional methods (e.g. EDM, laser) cannot be applied or are not productive enough.     

Characteristics of abrasive slurry jet micro-machining: A comparison with abrasive air jet micro-machining

Abrasive slurry jet micro-machining (ASJM) uses a well-defined jet of abrasive slurry to erode features in a solid target. Compared with abrasive water jet machining (AWJM), the present ASJM system operates at pressures that are roughly two orders of magnitude lower and uses a premixed slurry of relatively low concentration. The objective of the present study was to gain a better understanding of the mechanics of erosion in ASJM by comparing its performance in the micro-machining of holes and channels in borosilicate glass with that of abrasive air jet micro-machining (AJM), a process that is simpler and relatively well understood. A new ASJM system was developed and used to machine blind holes and smooth channels of relatively uniform depth that did not suffer from the significant waviness previously reported in the literature. The effect of particle velocity, particle concentration, jet traverse speed and jet impact angle were examined. A direct comparison of ASJM and AJM results was possible since novel measurements of the crushing strength of the aluminum oxide abrasive particles used in both experiments proved to be unaffected by water. Brittle erosion was shown to be the dominant material removal mechanism in both ASJM and AJM in spite of the significant flow-induced decrease in the local impact angles of many of the particles in ASJM. A new model of the rapid particle deceleration near the target surface helped explain the much smaller erosion rates of ASJM compared with those in AJM. The modeling of the erosion process during the micro-machining of channels showed that the effect of the local impact angle at the leading edge of the advancing jet was much more significant in ASJM than in AJM, primarily due to the narrower focus of the jet impact zone in ASJM. The differences in the water and air flow fields and associated particle trajectories were used to explain the steeper side walls and flatter bottoms of the holes and channels machined with unmasked ASJM compared to those with masked AJM. The respective structures of the water and air jets also explained the much sharper definition of the edges of these features using ASJM compared with maskless AJM. The results of the study show that ASJM can be used to accurately micro-machine channels and holes with a width of 350–500 μm and an aspect ratio of 0.5–1.3 without the use of masks.